1. Field of the Invention
This invention relates to flame retardant insulation compositions which are moisture curable. The compositions, which have a superior balance of processability and physical properties, comprise an ethylene-alkoxy silane copolymer, a halogenated anhydride and antimony trioxide.
2. Description of the Prior Art
Crosslinked ethylene polymers are used extensively for wire and cable insulation. Crosslinking improves the mechanical strength, heat resistance and other essential properties of the polymers.
One type of flame retardant crosslinkable system utilizes ethylene-vinyl acetate (EVA) copolymers with a silane compound and a chemical crosslinking agent, most commonly an organic peroxide. Compositions of this type are disclosed in U.S. Pat. Nos. 3,832,326; 3,922,442; 4,349,605; and 4,381,362. Hydrated organic filler(s) are added to impart the desired degree of fire retardance. Whereas the crosslinkable EVA compositions are widely used by the wire and cable industry, they are not without problems. Due to the presence of the chemical crosslinking agent during processing and extrusion, operating conditions are limited and must be carefully controlled to avoid premature crosslinking, commonly referred to as "scorching." Premature crosslinking results in poor extrudate quality. To minimize scorching processors often must use conditions which require them to operate at line speeds much below the maximum capabilities of the equipment. Post crosslinking in an autoclave to obtain reasonable cure levels may also slow down the wire coating process.
To achieve the degree of crosslinking necessary to develop optimal physical properties with the EVA systems, fairly rigorous cure conditions must be used. Typically, the products are cured using a continuous vulcanization (CV) line where the extruded wire or cable product is contacted with 400.degree. F., 200 psi steam in an autoclave. CV lines are expensive to maintain and operate.
Moisture curable compositions have been developed which eliminate the need for steam curing. These compositions utilize ethylene polymers which have alkoxy silane functionality incorporated into the polymer either by grafting an unsaturated alkoxy silane onto an ethylene polymer or by directly copolymerizing ethylene with an unsaturated alkoxy silane. Upon exposure to moisture, the alkoxy silane groups undergo hydrolysis/condensation reactions to form crosslinks. While compositions based on ethylene-alkoxy silane copolymers are essentially self-curing at room temperature, processors generally place the extruded wire or cable products in "cure rooms" maintained at high humidity and elevated temperatures, typically 100% humidity at 140.degree. F., for 12 to 24 hours.
Crosslinkable ethylene polymers prepared by grafting unsaturated alkoxy silanes to ethylene polymers are disclosed in U.S. Pat. No. 3,646,155. Crosslinkable ethylene polymers prepared by copolymerizing ethylene with an unsaturated alkoxy silane are disclosed in U.S. Pat. Nos. 3,225,018 and 3,392,156.
Ethylene-alkoxy silane copolymers can also undergo premature crosslinking, especially in the presence of silanol condensation catalysts which are typically used to promote the hydrolysis/condensation reactions. Premature crosslinking adversely affects (lowers) the melt index of the ethylene copolymer and, if this occurs to a significant degree, will result in poor processability and unsatisfactory extrudate surface appearance. In extreme cases, extrusion of the ethylene copolymer compositions becomes essentially impossible. Significant premature crosslinking can occur even in the absence of silanol condensation catalysts. The problem is particularly troublesome with filled compositions since particulate fillers can contain substantial amounts of moisture. This moisture is released during the mixing and blending operations and hydrolyzes at least some of the alkoxygroups of the alkoxy silane. The problems associated with filled ethylene-vinylalkoxy silane copolymers are widely recognized throughout the industry and are described in European Patent Application 89310667.4 published Apr. 25, 1990.
In view of the aforementioned problems, it is not possible to use hydrated organic fillers, such as hydrated aluminum oxide, as flame retardants for the ethylene-alkoxy silane copolymers. Halogenated compounds are, therefore, utilized for this purpose.
U.S. Pat. No. 4,397,981 discloses the use of organic halogen compounds, such as chlorinated polyethylene, to impart flame retardance to moisture curable copolymers of olefins and olefinically unsaturated silane compounds. Use is, however, limited to random copolymers. Undesirable premature crosslinking is obtained when the halogenated organic compounds are employed with graft copolymers. Even with random copolymers, certain halogenated compounds adversely affect processability and can result in extrudates with undesirable surface blemishes.
It would be highly useful if improved olefin/alkoxysilane copolymer flame retardant compositions were available. It would be even more advantageous if these flame retardant compositions employed halogenated compounds which could be used with both random and graft ethylene/alkoxysilane copolymers. It would be still more desirable if the halogenated compounds also promoted crosslinking of the resulting formulation. These and other advantages are realized with the compositions of the present invention which will be defined in more detail to follow.